The principle of interrogation of a SAW sensor based on a resonator is illustrated in FIG. 1.
The complete system is composed of an interrogation unit (itself composed of a transmitter part and of a receiver part) and of the SAW pressure sensor. The interrogation system together with the SAW sensor have an antenna adapted to the operating frequency band (ISM band 433 MHz, 868 MHz, 2.45 GHz, etc.) which enables wireless interrogation of the sensor to be effected.
The mode of interrogation is as follows:
The transmitter of the interrogation system 1 sends an interrogation signal to the SAW sensor at a frequency close to the resonance frequency of the sensor corresponding to the step represented schematically by the block B1:                the electromagnetic signal received by the antenna of the sensor is converted into a surface wave via an interdigitated comb structure which is charged up owing to the piezoelectric properties of the substrate used (quartz for example) corresponding to the resonator of the SAW type, step illustrated by the block B2, and this surface wave sees its properties modified as a function of the pressure and/or of the temperature which affect the conditions of propagation (in particular speed, which will affect the resonance frequency);        the sensor re-transmits an echo at its intrinsic resonance frequency which carries the information linked to the phenomenon that it is desired to identify: variation of temperature and/or of pressure, step B3;        outside of the time period for transmission, the receiver of the interrogation system 1 detects all or part of the echo from the SAW sensor and extracts from the response received the information sought that can relate to the temperature and/or the pressure, step B4.        
Generally speaking, the interdigitated comb structure may be incorporated between reflectors, creating a resonant cavity characterized by a certain resonance frequency. This frequency depends primarily on the speed of propagation of the waves under the array which itself mainly depends on the physical state of the substrate. It is therefore sensitive to the effects of stresses applied to the substrate. It is therefore from the resulting variation in frequency measured by the interrogation system that the state of stress in particular can be evaluated.
In the case of a stress measurement, for example an effect of hydrostatic pressure, a differential measurement may be effected. If two resonators designed to be compensated for the quasi-static thermal effects at the operating temperature of the sensor are used, of which only one of the two is subjected to the effects of stress, then the difference in frequency between the two resonators is representative only of the effects of stress. Such a measurement is thus referred to as a referenced measurement.
In the case of a pressure sensor using a reference, at least one of the resonators is sensitive to the pressure applied. This can be achieved by for example localizing the resonator on a thinned part of the piezoelectric substrate. Its frequency variation is linear as a function of the pressure (within the linear deformation limit):f(P)=f(P0)+Sp×P Sp (expressed in KHz per MPa for example) being denoted the sensitivity to pressure. The latter depends on the geometry of the aperture and on the properties of the piezoelectric substrate. P denotes the over-pressure (or under-pressure) with respect to the reference pressure P0.
According to the prior art, a pressure and temperature sensor can advantageously be based on a differential structure using three SAW resonators.
A first resonator, referred to as pressure resonator, uses the axis of propagation X, as does a second resonator RSAW, but is positioned in a thinned region in such a manner that when an over-pressure or an under-pressure (with respect to the pressure of the cavity) is exerted on the lower face of the device, the frequency of said first resonator varies proportionally. Said first resonator uses the same direction of propagation as the second resonator (same dependency of the frequency as a function of the temperature). The difference in frequency between the first and second resonators consequently allows information linked only to the pressure exerted on the lower face of the device to be obtained irrespective of the temperature.
The second resonator using the normal axis of propagation X is situated in a region exempt of stresses. A third resonator, also located in a region with no stresses, is inclined at a certain angle, typically 20°, with respect to the axis X. Inclining said third resonator gives the latter a different sensitivity versus temperature. The difference in frequency between the second and third resonators consequently allows information linked only to the temperature to be obtained irrespective of the state of the pressure exerted on the lower face of the device.
Using a differential structure offers the advantage of reducing the non-linearity of the sensor given that the residual non-linearities are corrected by the calibration of the sensor. Another advantage of the differential structure resides in the fact that a large part of the effects of aging may be overcome.
One exemplary embodiment of a SAW sensor for measuring pressure and temperature according to the state of the art is furthermore illustrated in FIG. 2. According to this configuration, the sensor of the quartz chip type (AQP for “All Quartz Package”) is composed of a base section 10 containing the SAW resonators: R1, R2 and R3 and of a cap 20 made of quartz rigidly attached to the base section via a fillet of glass paste. The base section is furthermore bonded onto a printed circuit 30 compatible with the temperatures of use, the whole assembly being enclosed by a protection cover 40 made of a plastic material.
The chip thus comprises the three resonators R1, R2 and R3. The resonator R1 is located on a thinned region (a membrane obtained by a process of mechanical machining for example) and its resonance frequency consequently depends on the external pressure and on the temperature. The external pressure is transmitted via an incompressible silicone gel 60 (for example) which fills the aperture.
The resonator R2 has a structure very close to that of the resonator R1 but is positioned in a region where the stresses associated with the pressure are negligible (non-thinned region). The resonator R2 in particular exhibits the same sensitivity to temperature as the resonator R1. The difference in frequency between the resonators R1 and R2 consequently supplies information which principally depends on the pressure.
The resonator R3 is also positioned in a region where the stresses associated with the pressure are negligible. The latter is inclined with respect to the resonators R1 and R2. This orientation endows it with a different sensitivity to temperature, in particular, with respect to the resonator R2. The difference in frequency between the resonators R2 and R3 consequently supplies information which depends only on the temperature.
This type of structure nevertheless has some drawbacks:                the latter requires an individual calibration of the sensors in pressure and in temperature owing to the technological dispersions in the processes of fabrication of the resonators and of machining of the quartz substrate in order to form the membrane;        the insensitivity to stresses associated with the pressure of the gas or of the fluid of the resonators R2 and R3 greatly depends on the assembly tolerances in particular on the bonding of the AQP chip;        the cost of the AQP chips is high owing to the complexity of the fabrication process. The latter indeed requires a first phase for machining away the capping wafers (which, thus rendered fragile, generate yield problems), a phase for deposition of glass paste on the capping wafers and a glass sealing process.        